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LE PUY-EN-VELAY, France (AP) — The battle for victory will go down to the wire at the Tour de France.

With just six stages left before the three-week race reaches the Champs-Elysees, only 29 seconds separate the top four riders in the general classification.

Defending champion and three-time winner Chris Froome has an 18-second lead over Fabio Aru of Italy. Frenchman Romain Bardet, the runner-up to Froome last year, is 23 seconds back from the leader, in third place. In fourth, is Colombian Rigoberto Uran.

It’s an unusual situation ahead of an intense final week of racing that includes two Alpine stages in high altitude and a short time trial.

“It’s the hardest fought battle in terms of Tours de France I’ve done before,” Froome said during Monday’s rest day. “I’m just grateful I’m on the right side of those gaps.”

Froome has the strongest team and remains the favorite to win in Paris, despite some rare signs of weakness.

He endured a bad day in the Pyrenees during a grueling stage to the ski station of Peyragudes, when he lost the overall lead to Aru after wearing the race leader’s yellow jersey for seven days. But the Team Sky leader recovered in style two days later, when Aru was trapped at the back of the peloton in Rodez.

On a windy day in the south of France, Froome and his teammates showed their superiority by riding at the front when the peloton stretched out and managed to put 24 seconds into their leader’s closest rival.

Froome’s main asset in the final stretch will be the strength of his teammates. Expect them to ride at the front in the mountains and to set a punishingly fast tempo – all designed to prevent others from attacking. If Froome is in form, he will be untouchable.

The collective strength of the Sky Team was on display Sunday when Froome was forced to change his rear wheel in the final 40 kilometers and got dropped.

“I was just standing there on the side of the road with my teammate Michal Kwiatkowski, trying to change wheels. I thought it was potentially game over for me,” Froome said.

But Kwiatkowski quickly handed over his wheel and Froome was helped back to the front by teammates Sergio Henao, Vasil Kiryienka and Mikel Nieve, erasing a 45-second gap.

Mikel Landa, who looks strong enough to be a leader in another team, was riding at the front but waited for Froome to catch up and the pair worked together to finish with the main contenders.

Froome has also showed great composure and calm when in trouble. In danger of losing the coveted leader’s jersey, he did not panic while Bardet, Uran and Aru failed to join forces at the front.

“I think Chris was strong because he was calm. The temptation can be to go too hard too quickly, you panic a little bit, go really, really deep to get on too quickly and of course you just explode,” Team Sky manager Dave Brailsford said.

Froome’s rivals have now to find a way to unsettle and isolate him in the Alps before Saturday’s time trial in Marseille, where the British rider will have the upper hand in the race against the clock.

Here is a look at the other contenders ahead of Tuesday’s Stage 16 (7:30 a.m. ET, NBC Sports Gold and NBCSN) that will lead the peloton to the gateway to the Alps in Romans-sur-Isere.

FABIO ARU

Aru was the big winner in Peyragudes, where he took the yellow jersey for the first time after Froome cracked in the last 500 meters of the stage.

But he could not build on the momentum and was isolated in the finale of the Rodez stage, handing overall lead back to Froome after only two days in yellow.

The former Vuelta champion is paying the price for his Astana team’s weaknesses. The Kazakhstan-funded team has lost key members Jakob Fuglsang and Dario Cataldo in crashes, and Aru has to count on his own skills when in trouble.

He will ride on his favorite terrain from Wednesday when the race enters the Alps. His only option if he wants to succeed Vincenzo Nibali on the list of Italian winners of the Tour will be to attack.

“This is going to be a very tough final week and so not everything is lost,” Aru said. “There aren’t just a few seconds difference now between me and Froome, but there aren’t so many either. So the Tour is still wide open.”

ROMAIN BARDET

Bardet has made no mistake so far in his bid to become the first Frenchman in 32 years to win cycling’s biggest race.

The 26-year-old climbing specialist finished runner-up to Froome last year and wants to move one step higher on the podium.

“I’m waiting for an opportunity to create a time difference,” Bardet said.

A very attack-minded cyclist with a natural instinct for racing, Bardet is in superb physical shape. In Peyragudes, his lethal acceleration earned him the stage win. He will try to reproduce the move in the mammoth Stage 18 to the Col d’Izoard, which features a final 14.1-kilometer ascent to the top of the mountain, at an altitude of 2,360 meters.

“I’m not thinking about the time trial,” Bardet said. “We’ll check the situation after the Izoard. I’m going to ride two Alpine stages as if they were classics.”

RIGOBERTO URAN

Twice a runner-up at the Giro, the Colombian from Cannondale-Drapac is now a serious contender for the overall win in France.

“We knew Rigo was super good and capable of winning mountain stages, and that he was capable of being in the top five overall,” said Jonathan Vaughters, manager of Uran’s Cannondale-Drapac team. “But that he was going to be 29 seconds behind? No.”

An excellent climber with good downhill skills, Uran has taken advantage of the very mountainous route in 2017. He made headlines during the first week when he won a stage in Chambery despite riding on a faulty bike, unable to change gears for the final 23 kilometers.

Uran, a former Team Sky rider, finished second at the Giro in 2013 and 2014 after winning the Olympic silver medal in the road race in London in 2012. He decided not to compete at this year’s Giro to arrive in perfect shape at the Tour, a good strategy so far.

Vaughters said Uran will not be content with a podium finish in Paris.

“That’s not really Rigos’s mentality, he is a kind of all-in guy,” Vaughters said. “For me, if we risk everything and he ends up in fifth place because we rolled the dice and it went the wrong way, c’est la vie.”